create save_instance method

[valdergallo]

Update save to get another method that could be override

def save_instance(self, instance, **data):
        record = instance(**data)
        record.save(force_update=False)
        return record

    def save(self, instance=None):
        """
        Save all contents
        """
        if not instance:
            instance = self.Meta.model

        if not instance:
            raise AttributeError('Invalid instance model')

        if self.Meta.transaction:
            with transaction.atomic():
                for row, data in self.cleaned_data:
                    _ = self.save_instance(instance=instance, **data)

                try:
                    self.pre_commit()
                except Exception as e:
                    self._error.append(self.get_error_message(e, error_type='__pre_commit__'))
                    transaction.rollback()

                try:
                    transaction.commit()
                except Exception as e:
                    self._error.append(self.get_error_message(e, error_type='__trasaction__'))
                    transaction.rollback()

        else:
            for row, data in self.cleaned_data:
                _ = self.save_instance(instance=instance, **data)

        self.post_save_all_lines()

        return True